Well I haven't gotten to the point where I actually decided what solution I'd actually use. Ideally, it would be a two arduino solution, where one would take care of building a complete "LocalTalk" packet, and SPI would be used to trigger an interrupt on the second arduino which would throw the IP stuff around that packet and send it to the ethernet controller. I have two arduinos already so this is a low-cost endeavour. Right at the moment, I'm trying to sell off some of my mac things to fund this project.